1. Field of the Invention
This invention relates to methods of purifying waste solutions containing electroactive species and more particularly to methods for purifying such solutions by electrolytic reaction using pulsed current electrolysis. The invention also relates to purification of metal-containing waste solutions by electrolytic deposition of the metal on porous electrodes having a high specific surface area using pulsed current electrolysis.
1. Brief Description of the Prior Art
Waste solutions containing pollutants that are electroactive species can be purified by an electrochemical process wherein the pollutants are destroyed or precipitated by oxidation or reduction at electrodes in contact with the solution.
Waste solutions containing metals in ionic form may be freed of the polluting metal ions by electrodepositing them as free metal at the cathode of an electrochemical cell. The major advantage of the electrochemical method for treating metal-containing waste water is that the metal ions can be recovered in metallic form, without the use of chemical reagents and without the generation of secondary wastes. However, electrochemical purification of metal-containing waste solutions has encountered certain difficulties due to the stringent limits on metal ion concentration that have been imposed by the ever-stricter legal requirements for purity of industrial effluent streams.
Other electroactive species found as pollutants in waste water, e.g., cyanide ions, can be removed by electrochemical oxidation at an anode.
The major challenge to electrochemical processing of low concentration waste water is the low current efficiency and high effluent concentration due to the hydrogen evolution side reaction. The extent of hydrogen evolution is dependent on the electrode potential, the hydrogen overpotential (.eta..sub.H) on the metal being deposited and the pH of the solution. The current efficiency for the common case of deposition of a metal ion may be defined as: EQU current efficiency=i.sub.M /(i.sub.M +i.sub.H)
where i.sub.M is the current density for metal deposition and i.sub.H is the current density for hydrogen evolution (A/dm.sup.2), and i.sub.M is defined as: EQU i.sub.M =nFk.sub.M C.sub.M
where C.sub.M is the concentration of metal ions (mol/L), k.sub.M is the mass transfer coefficient of the metal ion (dm/s), F is the Faraday constant (96,500 C/s) and n is the number of electrons involved in the reaction (eq./mol).
As is known to those skilled in the art, the potential required for deposition of a metal becomes more negative with decreasing metal concentration. For metal-contaminated waste water which must be remediated to low metal ion concentration the actual potential of deposition frequently becomes more negative than the potential for evolution of hydrogen. Consequently, the undesired evolution of hydrogen is likely to occur. Because of the hydrogen evolution reaction, the current efficiency will be relatively low when metals are recovered from dilute solution.
One way to minimize the effect of low metal ion concentration on the efficiency of the electrolytic process is to provide a high mass transfer rate. This is evident from a consideration of the concentration gradients produced in the solution by the electrolytic process. As the metal ions adjacent to the cathode are attracted to it and precipitated the concentration of the metal ions near the cathode is decreased, and the potential required for their deposition becomes more negative. In order to increase the rate of metal deposition the rate of mass transfer from the bulk of the solution to the depleted region adjacent to the electrode must be increased. Evidently the amount of metal ion in the volume adjacent to the cathode can be increased by increasing either the rate at which the ions are moved from the bulk solution to the near-electrode volume or by increasing the area of the electrode itself, thereby increasing the volume of the near-electrode layer. Merely increasing the area of flat plate electrodes would provide some benefit, but at the cost of increasing size and complexity of the electrochemical cell itself. Consequently, efforts at increasing the mass transfer rate in electrochemical cells have concentrated on using forced flow of electrolyte using an external pump, mechanically moving the electrode itself within the solutions, the use of turbulence promoting structures and conditions in flow systems, the use of stirring by gas sparging, and the use of three-dimensional electrodes to provide increased electrode surface area in a given cell volume.
A number of workers have investigated the electrochemical process of metal recovery with a view to improving its efficiency. Baily, D., et al., Plat. and Surf. Finish., 75 (4), p 26 (1988), used porous carbon fiber flow-through electrodes to increase the active surface area of the cathode and hence reduce the mass transfer limitations. However, the power consumption of such an arrangement is too great. In addition, as these flow-through electrodes become blocked with deposited metal, the buildup in pressure drop across the recovery unit can result in leakage and mechanical problems.
More recently, Walsh, F. C., and Gabe, D. R., Trans. Inst. Chem. Eng., 68, p. 107 (1990), approached the mass transport problem of metal recovery by working in a turbulent flow electrochemical reactor. Zhou, C. D., and Chin, D. -T., Plat. and Surf. Fin. 80, p. 67 (1993), investigated an electrochemical process for simultaneous metal recovery and cyanide destruction using a plating barrel-type cathode and a packed-bed anode. Due to the enhanced mass transfer rate induced by the motion of particles in the plating barrel, metal and cyanide concentration can be reduced to 1 part per million (ppm). However, in this work direct current (DC) electrolysis was used, and eventually, as the metal and cyanide concentration decreased to very low levels, the electrical energy consumption became excessive.
The approach of previous work, such as that described above, has been to circumvent the limitations imposed by mass transport requirements in an electrolytic cell by using a method based on fluid mechanics, i.e., generating turbulent flow by forced pumping, use of turbulence promoting structures, and motion of the electrode itself. It does not appear that enhanced mass transport by varying the electrochemical conditions of the electrolytic process has been used in attempts to remediate waste solutions to very low levels of metal ion concentration.
Pulsed current electrolysis of solutions containing relatively high concentrations of copper ions generated in production of copper by leaching of ores has been used for recovery of copper from the leaching solutions as disclosed in Pittman et al., U.S. Pat. No. 3,884,782. Pittman uses massive electrodes, e.g., stainless steel sheets, on which to plate out the copper rather than porous electrodes. The pulsed current is disclosed as increasing the purity of the deposited copper.
Accordingly a need has continued to exist for a method of increasing the efficiency of electrochemical remediation of waste solutions and particularly for a method of removing metals from dilute waste solutions by electrodeposition.